Most prior-art thermoplastic leaching chambers have a number of design characteristics in common, both for functional and manufacturing reasons. Typically, chambers have slotted, inwardly sloped, planar sidewalls, which run up to a curved arch top. They have arch-shape cross sections, and wide peak and valley corrugations running up over the arch. For example, see U.S. Pat. No. 5,017,041 of Nichols et al.
Slotted sidewall perforations provide open area, for infiltration of wastewater through the sidewall into the soil surrounding the chamber. Prior art chambers have relatively few corrugations, typically about one peak per foot, because that makes more area available for slot opening in peaks and in valleys which are usually the only areas with perforations. In use, leaching chambers must resist the loads from both overlying soil, and from vehicles and other things traveling along the soil surface, as well as lateral load of soil on the sidewall. Since the slots or other perforations weaken the sidewall, the sidewall is substantially thickened in vicinity of the slots, and ribs and other structures are provided for strength.
During use soil should not enter the chamber through the sidewall perforations. Some prior art devices simply have holes in thin walls, and geotextile, or porous fabric, laid over the sidewall prevents entry of soil. But that approach is undesired by many persons, because of cost and nuisance. The present invention is concerned with the class of chambers, which have perforations that are intended to inhibit soil entry by shape, without use of geotextile. The intent is that dimensions of the perforations, typically horizontal slots, themselves inhibit soil entry. Commonly, the portions of sidewall which are just above and below any slot are referred to as louvers. Louvers project from the basic sidewall and make slots deep compared to what their depth would be otherwise. But doing that increases wall thickness, which increases chamber weight and cost. In a typical chamber, the through-wall length of a slot might be increased to about 0.5 inch by louvers, where the basic wall thickness of the chamber elsewhere is about 0.13 inch. However, louvering increases the amount of material in a chamber, and requires substantial attention to get proper feeding during molding.
Leaching chambers must be reliably and economically fabricated, and nested for shipment. When injection molding is used, feeding of different regions, particularly louvers near slots, is accomplished by flowing plastic along ribs, which also strengthen the structure. Ribs usually run lengthwise and transversely on the interior and or exterior of a chamber. However, the presence of ribs lessens the ability to stack chambers in closely nested fashion. See U.S. Pat. No. 5,511,903 for information relating to chamber parameters and nesting. The result of the various trade-offs has been that a typical commercial slotted wall leaching chamber made of high density polyethylene is about 6 feet long, about 3 feet in width at the base, about 12–18 inch high. And it has five or six peak corrugations, louvers, ribs, and weighs 25–40 pounds or more.
The prior art chambers work well and have enjoyed commercial success. But there is a constant aim to improve chambers, so effectiveness or performance can be increased for the same cost, or so that cost can be reduced while maintaining effectiveness. One of the ways to reduce costs is to reduce the weight of plastic in a given size chamber, thereby reducing material and manufacturing cycle costs. Progress has been obtained in some prior art chambers by using gas assisted injection molding, wherein some interior portions are made hollow. See U.S. Pat. No. 5,716,163. Further improvements are desired.